Manufacturing method of semiconductor device

A manufacturing method of a semiconductor device, comprising:

• providing at least one mesa structure, wherein the at least one mesa structure comprises a III-V compound semiconductor layer;
• forming a passivation layer on the at least one mesa structure;
• forming a gate dielectric layer on the passivation layer;
• forming a gate electrode on the gate dielectric layer;
• and
• performing an etching process to the gate dielectric layer for thinning the gate dielectric layer before forming the gate electrode, wherein a thickness of the gate dielectric layer is greater than a thickness of the passivation layer before the etching process, and the thickness of the gate dielectric layer is less than the thickness of the passivation layer after the etching process.